1. Field of the Invention
The present invention generally relates to a frequency selection apparatus, a mobile communications system, and a multi-band frequency resource management method for realizing effective use of frequency resources, maintaining a service area, and improving throughput under an environment where two or more cellular systems using the same communication system in two or more frequency bands are in service.
2. Description of the Related Art
In recent years and continuing, demands for mobile communication services are rapidly increasing, and realization of high speed and large capacity communication services are called for, such as not only conventional voice and low speed data communications, but also moving picture and high-speed data transmissions. In order to meet these demands, it is necessary to use wide frequency band in mobile communications services that are capable of providing both high-speed (large capacity) and low-speed data communications.
However, radio frequency is a limited resource, and since it is difficult to obtain a wide band, communication systems that effectively use frequency are called for. Although radio communication systems have been advanced, are being diversified, and are offering various services, there are cases where a sufficient service area cannot be offered by a new system to be introduced.
Further, a radio communication system that uses two or more different frequency bands has been proposed, and put in practical use (for example, Non-Patent Reference 1), the radio communication system providing a wide service area and effective use of frequency.
Further, in PDC (Personal Digital Cellular), a system that is compatible with an 800 MHz and a 1.5 GHz band is available (for example, Non-Patent Reference 2). A base station and a mobile station according to this method are capable of standing-by, and communicating in the 800 MHz band and the 1.5 GHz band, wherein a frequency channel from one of the 800 MHz band and 1.5 GHz band is assigned. In this way, congested traffic of the 800 MHz band is distributed to the 1.5 GHz band.
Further, the frequency band at which the mobile station stands by is assigned based on information about a “band shift probability”, the information being provided by the network. A frequency channel that is available to the mobile station is obtained by the mobile station measuring the received signal strength of both frequency bands when a voice communication is to be carried out such that a suitable frequency band that can satisfy a desired quality is selected, and the communication is carried out.
Further, in Europe, in an attempt to smoothly introduce a W-CDMA (Wideband Code Division Multiple Access) system, a mobile station compatible to both W-CDMA and GSM (Global System for Mobile Communications) is being studied, and an experimental verification of connecting the compatible mobile station to a GSM and a W-CDMA network is also being carried out (for example, Non-Patent Reference 3).
A handover process from W-CDMA to GSM is as follows. In the case that a mobile station is communicating by using a switched line of a W-CDMA system, when received signal strength becomes less than a threshold, the network of W-CDMA directs the mobile station to measure a GSM signal. If the mobile station finds a GSM base station, a signal of which is greater than the threshold of received signal strength, the handover from W-CDMA to GSM is started.
Then, a RAN (Radio Access Network)
Controller determines whether a resource is available at the GSM base station. If the determination is affirmative, i.e., if there is a resource that can be assigned, the GSM base station transmits a handover command.
When the mobile station receives the handover command, connection to the GSM base station is carried out. As a result, a radio connection in agreement with the command is established. Then, the mobile station transmits a handover completion message to the GSM base station, connection with a GSM network is started, and the connection with the W-CDMA network is released.
A study is also being made about controlling two or more communication systems. There, a mobile station is controlled so that it is connected to one of the communication systems. In this way, system throughput is raised compared with the case where the mobile station is capable of communicating in only one communication system (for example, Non-Patent Reference 4 reference).
Further, studies are being made about methods to improve the throughput (for example, Non-Patent Reference 5). An example is a method wherein resources of two or more systems using the same communication parameter are independently controlled such that loads are properly distributed between the systems. Another example is a method wherein each mobile station corresponding to each system selects a system that provides the greatest throughput where two or more systems each having different transmission speed, number of channels, area composition, and propagation property are available.
By controlling two or more radio systems as described above, effective use of frequency resources, obtaining a service area, and improvement in the throughput are attained.                [Non-Patent Reference 1] R. Heichkero, et al., “Ericsson Seamless Network”, Ericsson Review, No. 2, pp. 76-83, 2002        [Non-Patent Reference 2] Yeshiva, Fukazawa, and Masuda, “Special Issue: 1.5 GHz band common method, System Outline”, NTT DoCoMo Technical Journal, Vol. 10, No. 1, pp. 6-14, April 2003        [Non-Patent Reference 3] G. Alsenmyr, et al., “Handover Between WCDMA and GSM”, Ericsson Review, No. 1, pp. 6-11, 2003        [Non-Patent Reference 4] A. Tolli and P. Hakalin, “Adaptive Load Balancing between Multiple Cell Layers”, proceedings of VTC Fall 2002, Vol. 3, pp. 1691-1695, September 2002        [Non-Patent Reference 5] T. Shono, K. Uehara, and S. Kubota, “Proposal for System Diversity on Software Defined Radio”, IEICE Trans. on Fund., Vol. E84-A, No. 9, pp. 2346-2358, September 2002        
[Description of the Invention]
[Problem(s) to be Solved by the Invention]
However, there are the following problems in the conventional technologies described above.
In the control system described above, control is performed based on measurement of a received signal, wherein difference in the frequency characteristic of the signal is not taken into consideration.
When selecting a system out of systems that use different frequency bands, and switching is to be carried out in an actual system, it is necessary to consider differences in propagation properties, fading influences, and the frequency bandwidth that can be obtained in real environments.